1. Field of the Invention
The invention relates generally to an apparatus, and a corresponding method, for chemically etching copper foil-clad substrates, such as copper foil-clad printed circuit board substrates.
2. Description of the Related Art
Thin conductive metal foils, such as thin copper foils, play an important role in the fabrication of printed wiring circuits on printed circuit boards. That is, such printed wiring circuits are typically formed by attaching, e.g. , laminating, a thin copper foil to a printed circuit board substrate. The thin copper foil is subsequently, for example, selectively etched to form the desired circuit pattern. This is accomplished by initially forming a patterned photoresist on the copper foil and then etching the copper foil while using the patterned photoresist as an etch mask. Alternatively, a circuit pattern is formed by selectively depositing additional copper onto the copper foil. In this alternative, a patterned photoresist is also initially formed on the copper foil. Then, additional copper is selectively deposited, e.g., electroplated, onto the copper foil, portions of which copper foil and/or deposited copper are subsequently etched away, and thus sacrificed.
Regardless of whether selective etching or selective deposition is used, it is often important that the above-described copper foil be as thin as possible. For example, if selective etching is used, then the thinner the copper foil, the more closely the etched copper foil will match the patterned photoresist. On the other hand, if selective deposition is used, where the thin copper foil is ultimately etched away, it is important that the etching of the copper foil not damage or degrade the corresponding circuit pattern, which is more readily achievable if the copper foil is truly very thin.
Currently, copper foils having thicknesses as small as 12 micrometers are used in forming printed wiring circuits. However, there is a strong desire in the printed circuit board industry to use even thinner copper foils, e.g. , copper foils having thicknesses of 9 micrometers, 5 micrometers or even 3 micrometers.
Various attempts have been made to form copper foils having thicknesses equal to or less than 9 micrometers on printed circuit board substrates. In several such attempts, the copper was sputtered directly onto the substrate. Unfortunately, the heat of the sputtering process was so great as to destroy, deform or undesirably alter the corresponding substrates.
Other attempts to form very thin copper foils have involved chemically treating substrates and then chemically depositing very thin copper foils onto the treated substrates. These attempts proved to be very sensitive to contamination, which led to pin hole voids in the deposited copper. Such pin hole voids are undesirable because they can and do degrade the electrical performance of the corresponding printed circuit board.
Significantly, a new method for forming a very thin copper foil on a substrate is described in U.S. Pat. No. 4,917,758 which issued to K. Ishizuka et al on Apr. 17, 1990. Here, a substrate bearing a relatively thick copper foil is chemically etched with a spray-type etching machine in order to thin the relatively thick copper foil. While the reported variation of the thickness of the copper foil remaining after the etching is within +/-2.0 micrometers, the etch rate is less than or equal to 0.3 micrometers per second. Such low etch rates require corresponding etch times which are much too long for many commercial applications.
Thus, those engaged in the development of printed circuit boards have sought, thus far without success, apparatuses and methods for effectively forming very thin copper foils within commercially viable time periods.